1. Field of the Invention
The present invention generally relates to a vehicle steering wheel structure. More particularly, the invention relates to a vehicle steering wheel structure having a highly efficient vibration reducing effect.
2. Description of Related Art
In order to effectively reduce vibration of a steering wheel incorporated into a vehicle, it is necessary to provide a large (heavy) damper mass in the inner space of a rim portion of the steering wheel. The inner space of the rim portion of the steering wheel is very narrow because an airbag device, a horn component, a harness and the like are incorporated therein. When a damper mass is provided in a narrow space, it is difficult to secure a space for incorporating a large damper mass. Therefore, it is reasonable to arrange a plurality of small damper masses obtained by dividing the large damper mass, in a distributed manner. By doing so, it is possible to incorporate damper masses having a required weight into the narrow inner space of the rim portion.
Japanese Patent Application Publication No. 2002-145075 and Japanese Patent Application Publication No. 2002-323087 disclose vehicle steering wheel structures in which a plurality of damper masses for reducing vibration of a steering wheel is arranged so as to be distributed in the inner region of the rim portion of the steering wheel.
The “vibration damper for steering wheel” disclosed in Japanese Patent Application Publication No. 2002-145075 aims to provide a vibration damper for a steering wheel that is capable of reducing major vibration of the steering wheel by damping vibration in a rotational direction, in particular as well as vibration in a direction vertical to the direction of a steering shaft of the steering wheel. According to the vibration damper, a damper that includes a housing having an inner space and a weight inserted in the inner space of the housing in a non-bonded manner with a gap in relation to the housing is attached to a portion of a steering spoke portion near the steering wheel.
The “damping device” disclosed in Japanese Patent Application Publication No. 2002-323087 aims to provide a low-cost steering wheel damping device that can be easily manufactured and attached with a simple configuration. According to the damping device, a mass body is accommodated with a gap in a container-shaped holding portion that is molded integrally with a core bar or a lower cover of a steering wheel, and impact dampers covered with a cover member that is integrated with a switch attachment member are disposed symmetrically at the left and right positions of a boss portion. With such a simple configuration using existing members, the vibration reducing effect of the steering wheel can be easily obtained.
The individual small distributed damper masses cannot sufficiently damp vibration and it is not possible to secure such a vibration reducing effect as expected when a large damper mass is provided.
In order to secure the same vibration damping effect as when a large damper mass is provided, it is necessary to increase the weight of the individual small damper masses. When a plurality of small damper masses is arranged, it is not possible to obtain a sufficient vibration damping effect with the mass corresponding to one large damper mass. If the weights of the individual small damper masses are increased, the total weight becomes larger than that when a large damper mass is provided. Moreover, if the weight is increased, the dimensions increase although the damper masses are small, and the required installation space also increases. The problem of the increase in space can be solved when the material of the damper mass is changed to special material having larger specific gravity than cast iron or the like that is generally used. However, there is another problem in that the cost may increase if special material is used.